refactor: clarify waiter arena contracts

[tisonkun]

Summary

• represent reusable arena slots with SlotId, removing manual encode/decode calls and the associated unsafe construction
• represent a caller-owned WaitSet entry with the non-Copy WakerToken, and use slot/token consistently through its callers
• document the arena free-list, slot-ID lifetime, and wake-epoch invariants while preserving next_vacant as the field name
• make the arena module and inline-one value holder private, with the sibling-facing arena API declared pub inside that module boundary
• hide ArenaValues behind owning iterators from Arena::take_all and WaitSet::take_wakers
• retain option-niche size checks without imposing unnecessary repr(transparent) layout guarantees on internal wrappers
• cover repeated unlinking of detached waiters, middle-waiter removal with slot reuse, and stale Waker tokens across wake epochs

The existing module-level allow(dead_code) remains intentionally simple: WaitList and WaitSet use different subsets of the arena API, so single-primitive feature builds inevitably leave part of it unused.

Small-vector decision

ArenaValues is the safe, specialized equivalent of the small-vector behavior needed here: keep one value inline, store only additional values in a Vec, and consume the values through iteration. I evaluated vendoring SmallVec 1.15.2, but its general implementation and unsafe storage machinery are disproportionate to these three operations. This keeps the focused representation without adding a production dependency or vendored third-party surface.

Validation

• cargo +1.86.0 check --package asyncband --all-features --tests
• cargo test --workspace --all-features
• cargo run -p x -- check
• cargo run -p x -- lint